[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM14-P19] 1-D analysis of broadband magnetotelluric data around the epicenter of the 1997 Kagoshima earthquake doublet

*KEITA Matsunaga¹, Koki Aizawa², Hiroki Ogawa³, Mitsuru Utsugi⁴, Ryokei Yoshimura⁵, Ken'ichi Yamazaki⁵, Kazunari Uchida² (1.Department of Earth and Planetary Sciences, Graduate school of Science, Kyushu University, 2.Institute of Seismology and Volcanology, Kyushu University, 3.Tono Geoscience Center, Japan Atomic Energy Agency, 4.Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 5.Disaster Prevention Research Institute, Kyoto University)

Keywords: magnetotelluric, the 1997 Kagoshima earthquake doublet, resistivity structure

The 1997 Kagoshima earthquake doublet, which occurred on March 26 (M6.5) and May 13 (M6.3), has several interesting features (e.g., two large earthquakes on the region with no significant surface fault, the formation of an F-shaped aftershock zone, uneven fracture extension, etc.). Umeda et al. (2014) have conducted broad-band MT observation around the hypocenter region, and imaged the near-vertical conductive zone with a width of 20 km. They suggested that the deep fluids contributed to the occurrence of the 1997 earthquake. The studies on the southern Kyushu region show the seismic low-velocity zones (Sadeghi et al. 2000), high seismic attenuation zones (Saita et al. 2015), and low resistivity zones (Hata et al. 2017, 2020), which all suggested the presence of fluids around the focal area. Moreover, the occurrence of deep low-frequency earthquakes at approximately 15 km west of the hypocenters of the main shocks also suggests the presence of fluids. However, the relationship between fluids and the earthquake is not fully investigated. Our purpose is to understand why the above mentioned interesting features are shown on the 1997 Kagoshima earthquake. For this purpose, estimating of the high resolution resistivity structure is useful.
The MT method uses natural electromagnetic fields to estimate the electrical resistivity structure of the Earth's interior. Kyushu University and Japan Atomic Energy Agency (Umeda et al., 2014) have acquired broad-band MT data at 68 sites in total. In this study, as a first step of reanalysis of these data, we conducted one-dimensional analysis. We discuss the relationship between the preliminary resistivity structure and the seismic activity of the 1997 northwestern Kagoshima Prefecture earthquake. Future plan for additional MT observation is also shown.

References
Umeda, K., Asamori, K., Makuuchi, A., Kobori, K., 2014. Earthquake doublet in an active shear zone, southwest Japan: Constraints from geophysical and geochemical findings. Tectonophysics 634 (2014) 116–126